1. Field of the Invention
The present invention relates to an optical recording medium. More particularly, the present invention relates to an optical recording medium for multiple-recording by using photochemical hole burning.
2. Description of the Related Art
Hitherto, it is known that information can be recorded in an optical recording medium by using photochemical hole burning (hereinafter referred to as PHB).
An optical recording medium, which has an optical recording layer, employing this PHB method, is used in which host molecules, such as a high polymer, such as polymethyl methacrylate (hereinafter referred to as PMMA), are dissolved in a solvent, such as alcohol, and guest molecules, such as phthalocyanine (hereinafter referred to as H.sub.2 PC) or quinizarin (hereinafter referred to as DAQ), are diffused in a diluted condition, this solvent then being applied to glass or a high polymer film and dried [IBM Journal Research Development (J. RES. DEVLOP.), Vol.26, No. 2, pp.198-207, (March, 1982)].
When the recording medium is maintained at an extremely low temperature, for example, a liquefied helium temperature (4.2 K.), the energy of specific guest molecules is frozen to a specific energy state determined by an interaction with the host molecules specific to the guest molecules, thus permitting their energy state to be distinguished from the energy state of the other guest molecules. As a result, the wavelength position of an absorption spectrum of the specific guest molecules is fixed at a specific position. In this condition, if a light beam of a specific wavelength (.lambda.n) is spread on the recording medium, only the guest molecules having absorption at that wavelength position absorb light and move to another energy state. As a result, the absorbance of the recording layer in the specific wavelength (.lambda.n) is decreased, causing a hole at the position of (.lambda.n) in the spectrum. The use of the presence and absence of this hole as binary coded signals of 0 and 1 permits optical recording (see FIG. 6). If two or more kinds of wavelengths (.lambda.n) of a light beam used for recording are used, recording at each wavelength, that is, what is called multiplerecording in terms of wavelength dimensions, can be performed (see FIG. 7).
However, in the above-described conventional optical recording medium, that is, an optical recording medium in which DAQ or the like is used for guest molecules and PMMA or the like is used for host molecules, the wavelength region of a laser for causing holes does not match a semiconductor laser wavelength region (see FIG. 8). Therefore, a dye laser cannot choose but be used at present. A dye laser is generally large in construction and is not easy to use, and maintenance thereof is not simple.
Therefore, when an optical recording system is constructed, a laser, such as a semiconductor laser which can be easily systemized, should preferably be used.